The shelves have many books on them.
Answers
Explanation:
Hot air rises. Enough hot air will gently lift the envelope, gondola and passengers into the sky. Pilots can control their ascent and descent by heating more air with "burners" or slowly releasing air allowing the heated air to cool off or by using a variety of vents located in strategic points on the envelope vent out the hot air. A new commercial-made hot air balloon will cost in the neighborhood of $30,000 to $40,000. The propane used to fuel the burner which heats the air inside the envelope generally costs $30-$40 and lasts for one to two hours of flight. The ideal time for balloons to fly is typically in the morning, when the cooler temperatures and lower winds provide a better environment for flying. Balloonists prefer winds to be ten miles per hour or less.